The invention relates to a tandem master cylinder for a hydraulic dual-circuit brake system of a road vehicle with static brake circuits. Such a brake system includes primary and secondary outlet-pressure spaces which are assigned individually and in which brake pressure can be built up as a result of the displacement of a primary piston, controlled by pedal force, and of the displacement, co-related to the displacement of the primary piston of a secondary piston designed as a floating piston.
Tandem master cylinders of this type are generally known (see Alfred Teves GmbH, Brake Handbook, 9th edition, 1986, page 104, Bartsch-Verlag, Ottobrunn near Munich) and are offered, together with suitable brake boosters, as brake units for the widest variety of vehicle types by well-known brake manufacturers.
In such brake units which are installed in the engine space of a vehicle, in particular their constructional length is an important parameter and their limitation, which of course must not be at the expense of the functional reliability of a brake unit, is an important precondition for the construction of tandem master cylinders. Even only a slight reduction in the constructional length of, for example, a few millimeters is desirable and a reduction of the constructional length of, for example, 10 mm is viewed as a considerable technical achievement.
In tandem master cylinders of the known type, the pistons limiting the primary outlet-pressure space and the secondary outlet-pressure space are urged by restoring springs into their respective basic positions corresponding to the non-actuated state of the brake system and are characterized by the contact of these pistons against stops fixed relative to the housing. In this arrangement the stop determining the basic position of the secondary piston is designed as a pin fixed relative to the housing and passing radially through a longitudinal slot of an intermediate piece of the secondary piston connecting two flanges of this piston to one another. These flanges are arranged at an axial spacing from one another, one flange forming a movable limitation of the secondary outlet-pressure space, axially limited fixedly relative to the housing by an end wall of the cylinder housing, and the other flange forming a movable limitation of the primary outlet-pressure space relative to a follow-up space extending between these two piston flanges of the secondary piston and maintained in constant communicating connection with the brake-fluid storage tank. The restoring spring urging the secondary piston into its basic position is supported fixedly relative to the housing on the housing end wall limiting the secondary outlet-pressure space in the axial direction and engages on the piston flange of the secondary piston axially limiting the secondary outlet-pressure space. In this design of the master cylinder, in order to minimize its constructional length, the piston cross-sections and therefore their strokes and the restoring springs are coordinated with one another in such a way that, in a full-braking situation in which the maximum possible obtainable brake pressures are utilized, the master-cylinder pistons each executing their maximum stroke, the primary piston comes in contact against a supporting projection pointing towards the latter and located on the secondary piston and the latter, by means of a supporting projection pointing towards the end wall of the housing, comes in contact against this supporting projection or into the immediate vicinity of it and in these end positions of the pistons the two restoring springs are compressed to a block, these two supporting projections being used respectively for centering those portions of the restoring springs surrounding them coaxially.
A reduction in size of a tandem master cylinder of this type seems possible, at most, if the block lengths of its restoring springs, determining the minimum axial extents of the primary and the secondary outlet-pressure spaces of the tandem master cylinder, were reduced, although this could be achieved only by making the restoring springs weaker. However, this would necessarily entail an impairment of the functional properties of the tandem master cylinder, which would be unacceptable and, moreover, would also result in only a relatively insignificant reduction of the constructional length.
An object of the invention is, therefore, for a tandem master cylinder of the type mentioned in the introduction, to provide a design in which it is possible to produce the tandem master cylinder with a clearly reduced constructional length, without loss of functional reliability.
According to the invention, this object is achieved by configuring the restoring springs and the secondary piston so that the restoring springs extend concentrically to one another and thereby limit the overall length of the master cylinder arrangement.
The restoring spring urging the secondary piston of the master cylinder according to the invention into its basic position is transferred into a central inner space of the latter, with the two restoring springs being arranged coaxially in portions. The constructional length of that part of the cylinder housing limiting the secondary outlet-pressure space can thereby be reduced by approximately the compressed block length of the restoring springs otherwise arranged in the secondary outlet pressure space, thus making it possible to obtain a saving of constructional length of approximately 10 mm in certain preferred embodiments, that is to say an appreciable amount which must therefore also be considered a clear technical advance.
The restoring spring arranged within the secondary piston can at once be made sufficiently strong to generate the necessary pre-stress, as a result of which the secondary piston moves back into its basic position sufficiently quickly when the actuation of the brake system is cancelled.
Because, according to certain preferred embodiments of the invention, the restoring spring is supported on the stop piece fixed relative to the housing, via a sliding piece which can jointly execute rotational movements of the spring end facing the stop which occur both during the compression and during the relaxation of the spring, torsional stresses of the restoring spring, which could otherwise lead to lateral deflections of the latter and consequently to frictional losses, are as it were "compensated" in a simple way.
In a design provided according to certain preferred embodiments of the invention, a central valve causing the appropriate opening or shutting-off of the compensating flow path required for the secondary outlet-pressure space of the tandem master cylinder can be obtained without any appreciable contribution to the total constructional length of the tandem master cylinder, additional overflow cross-section being produced by means of a central passage channel located in the sliding piece.
The features of certain preferred embodiments provide an arrangement of the brake-pressure outlet of the secondary outlet-pressure space which is advantageous for producing the tandem master cylinder according to the invention with the shortest possible constructional length.
For producing a tandem master cylinder according to the invention, its design as a step cylinder according to certain preferred embodiments is especially advantageous, since in this case the portionally coaxial arrangement of the two restoring springs inside and outside the supporting projection of the secondary piston can be obtained especially simply in terms of construction.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.